Exhaust gas emitted from diesel engines contains particulate matter and constitutes one of the causes of local air pollution. Reduction of particulate matter emissions is one of the major environmental issues for which a swift solution is desired.
In order to reduce particulate matter emissions, much research is being devoted to studying systems wherein a particulate matter filter is situated in the diesel engine exhaust path to collect particulate matter in exhaust gas, and the collected particulate matter is then ignited and combusted. However, the particulate matter collected on the particulate matter filter does not ignite unless heated to a temperature of above approximately 600° C., whereas diesel engine exhaust gas temperatures are usually much lower than 600° C., and therefore the ignition temperature of the particulate matter must be lowered in order to achieve combustion removal of the particulate matter in the exhaust gas flow.
Incidentally, it has conventionally been known that catalysts carried on particulate matter filters can reduce the ignition temperature of the particulate matter, and for example, Japanese Examined Patent Publication HEI No. 7-106290 discloses a particulate matter filter carrying a mixture of a platinum-based metal and an alkaline earth metal oxide. With this particulate matter filter, the particulate matter ignites at a relatively low temperature of about 350–400° C., and is subsequently combusted in a continuous manner.
However, since the exhaust gas temperature during actual operation of a diesel engine is often below 350° C., it is impossible to achieve complete combustion removal of particulate matter at the aforementioned ignition temperature of about 350–400° C., and therefore pressure loss of the exhaust gas through the filter increases, creating the problem of fuel efficiency reduction. Moreover, combustion of abundant residue of particulate matter leads to exposure of the particulate matter filter to temperatures of above approximately 800° C., resulting in problems such as more rapid deterioration of the particulate matter filter.
It is therefore an object of the present invention to provide a particulate matter purifying catalyst which can easily oxidize particulate matter collected on particulate matter filters at a much lower temperature than is possible by the prior art.